1. Field of the Invention
The invention relates generally to temperature measuring devices and, more particularly, to an electronic device for accurately measuring temperature using a resistance-type sensor and developing a linear relationship between resistance and temperature.
2. Description of the Prior Art
Precision thermometry is needed in many areas of research and process control. One area of research is the study of ice formation in rivers and streams, particularly frazil formation. It is well known that supercooling of water during frazil formation rarely exceeds 0.03.degree. C., and never exceeds 0.05.degree. C. An instrument capable of measuring temperature within at least 0.01.degree. C. is required for an understanding of frazil formation Many other ice formation phenomena are also critically dependent on small changes of supercooling.
Known electronic thermometers generally use either a thermistor or a platinum resistance temperature device as the temperature sensor. Both are widely used and are known for their long-term stability. Thermistors, however, exhibit a much larger change in resistance per unit change of temperature, usually on the order of several Ohms per degree. This characteristic makes it easier to measure small changes in temperature. Because of its stability, durability and sensitivity, the thermistor is well suited for use in an electronic thermometer for the purposes noted above.
Two methods are commonly used to precisely measure resistance of the above thermal sensors the Wheatstone bridge, in which the unknown resistance is matched to a known resistance; and passage of a constant current through a thermistor while measuring the voltage across it, and calculating the resistance using Ohm's law. While these methods are well proven, they are inconvenient for determining temperature, especially in the field. First, the thermistor's resistance must be determined. Then the corresponding temperature is determined from a special table correlating resistance to temperature. Thus, a table must be available for each thermistor being used.
As examples of the prior art, Scott (U.S. Pat. No. 4,060,715), discloses a modified linearized bridge circuit for a sensor, which may be a platinum element, presenting an electrical resistance that relates to temperature by a second order polynomial, in which the conventional, manually-adjustable variable resistor is replaced by a feedback network having an operational amplifier.
Lamb (U.S. Pat. No. 3,934,476) discloses a circuit for an electronic thermometer in which one or more semiconductor diodes are connected in series with a thermistor to obtain a voltage nearly linearly related to temperature. The bridge circuit includes a reference resistance in series with a like number of identical diodes.
Dahlke (U.S. Pat. No. 4,205,327) provides a circuit for correcting the nonlinear output of a sensor, such as a temperature-sensing platinum resistive element. Nonlinear compensation is achieved by employing a non-linear sensor network to adjust a current source used to excite a bridge in which the sensor is connected, as a function of the sensed variable. The adjusted current offsets the output signal from the bridge to compensate for the sensor's nonlinearity.
The foregoing patents illustrate another shortcoming of prior-art electronic thermometers. With the circuits designed to accommodate a specific temperature sensor, changing the sensor requires modifying the circuitry. There are no provisions for interchanging sensors and quickly modifying the instrument to work with the new sensor
Furthermore, none of the available temperature measuring devices that approach the required accuracy and resolution are suitable for field use. Typical laboratory grade electronic thermometers have accuracies around 0.02.degree. C. to 0.04.degree. C. Most of the instruments require an AC power source, and almost all are limited to use at room temperature for correct operation. None of the available units are sufficiently rugged, waterproof or portable to permit their use outdoors, under extreme climatic conditions.